1. Field of the Invention
The present invention relates to the field of fabrication of integrated circuits. More specifically, the present invention relates to a process for improving the thermal stability of low dielectric constant materials included in integrated circuits.
2. Description of the Related Art
Modern integrated circuits are generally made up of millions of fit active and passive devices such as transistors, capacitors, and resistors disposed in a silicon wafer. These devices are initially isolated from one another, but are later interconnected together by an interconnect system to form functional circuits. The quality of the interconnection of these devices drastically affects the performance and reliability of the fabricated integrated circuit.
An interconnect system typically includes metal lines, spaced apart from each other. The metal lines interconnect the various active and passive devices found in a silicon wafer onto which an interconnect system is deposited and fabricated. The metal lines are separated by insulating dielectric material for isolating the metal lines from one another. Inherent in the structure of the interconnect system is a capacitance associated with the metal lines spaced apart from each other. Decreasing this capacitance is desirable as several advantages can be achieved therefrom, such as reduced RC delay, reduced power dissipation, and reduced cross-talk between the metal lines. The capacitance is inversely proportional with the distance between the metal lines. Thus, one way to reduce the capacitance between the metal lines would be to increase the space between the lines. However, this option is not desirable because of the limitations imposed by packing density.
Another way to reduce the capacitance between the lines of an interconnect system is to reduce the dielectric constant (k) of the dielectric material deposited between the metal lines. The capacitance of the metal lines is directly proportional to the dielectric constant of the dielectric material between the metal lines. The dielectric constant of a material is generally defined as the material""s ability to maintain a difference in electrical charge over a specified distance.
One dielectric material typically used to isolate metal lines from each other is silicon dioxide (SiO2). SiO2 is a thermally and chemically stable material. The dielectric constant of SiO2 is approximately 4. The dielectric constant is based on a scale where 1.0 represents the dielectric constant of a vacuum. Various materials exhibit dielectric constants from approximately 1.0 to values in the hundreds.
The dielectric constant of SiO2 is considered high. Recent attempts have been made to use low dielectric-constant materials such as organic and inorganic polymers that have densities and dielectric constants lower than those of SiO2 to replace SiO2 as a dielectric material thereby reducing the capacitance between the metal lines.
One such low dielectric constant material is fluorinated amorphous carbon (a-C:F) that has a dielectric constant of about 2.3 (xc2x10.4) and low thermal stability. This thermal stability is lower than the thermal stability of previously used oxides such as SiO2. Low thermal stability causes problems in integrated circuit fabrication, as during the process of fabrication of integrated circuits and their interconnect structures, temperatures in excess of 400xc2x0 C. are often reached. As processing temperatures climb, a-C:F materials heat up and decompose into highly corrosive fluorine species such as F, CF, CF2, and CF3. When these fluorine species outgas or outdiffuse into the surrounding metal layers, they tend to cause corrosion of the metal layers of the interconnect and create potential adhesion problems with juxtaposed dielectric films. These films may ultimately cause yield and reliability problems in the devices being fabricated.
It is desirable to provide a thermally stable a-C:F material and a method to improve the thermal stability by decreasing fluorine its outdiffusion from a-C:F during high temperature processing.
The present invention provides a process for forming a thermally stable low dielectric constant material. A gas mixture is prepared to form a fluorinated amorphous carbon (a-C:F) material. The gas mixture is mixed with a boron-containing gas.